Greenhouse Gas Sequestration In

was initiated to investigate the technical Greenhouse gas sequestration and economic feasibility of CO2 storage in a partially depleted oil reservoir (Government of Canada, 2000). The IEA in abandoned oil reservoirs: Weyburn project is exploiting EnCana Corporation’s $1.5 billion, 30-year com- The International Energy Agency mercial CO2 enhanced oil recovery operation, which is designed to recover an incremental 130 million barrels of oil Weyburn pilot project from the Weyburn field through the injection of gaseous CO2 under pressure. D.J. White, Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario K1A 0E9, Specifically, the IEA Weyburn Project Canada, [email protected] aims to comprehensively monitor and verify the progress of the CO flood G. Burrowes, EnCana Corporation, 421 7th Avenue SW, P.O. Box 2850, 2 and establish the likelihood of safely Calgary, Alberta T2P 2S5, Canada storing the CO2 in the reservoir for the T. Davis, Colorado School of Mines, Golden, Colorado 80401-1887, USA long term. Toward this end, a multidis- Z. Hajnal, Department of Geological Sciences, University of Saskatchewan, ciplinary, integrated program has been Saskatoon, Saskatchewan S7N 0W0, Canada formulated to address critical issues central to safe and cost-effective, long-term K. Hirsche, Hampson-Russell Software, 715 5th Avenue SW, Calgary, storage of CO . In this article, we focus Alberta T2P 2X6, Canada 2 on the regional geoscience framework I. Hutcheon, Department of Geology and Geophysics, University of Calgary, and the monitoring and verification Calgary, Alberta, T2N 1N4, Canada components of the project. E. Majer, 90-MS1116, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA GEOLOGICAL SETTING AND HISTORY OF THE WEYBURN FIELD B. Rostron, Department of Earth and Atmospheric Sciences, University of Alberta, The Weyburn oil field is located Edmonton, Alberta T6G 2E3, Canada southeast of Weyburn, Saskatchewan, S. Whittaker, Saskatchewan Industry and Resources, 201 Dewdney Avenue E, within the north-central Williston Basin, Regina, Saskatchewan S4N 4G3, Canada which contains shallow marine sedi- ments of Cambrian to Tertiary age (Fig. 1). The Weyburn field, which covers ~180 km2, was discovered in 1954 and ABSTRACT INTRODUCTION hosted an estimated 1.4 billion barrels of Carbon dioxide sequestration in geo- Carbon dioxide is the primary anthro- oil. Primary production within the field logical reservoirs is being evaluated pogenic greenhouse gas in the modern- continued until 1964, when the initiation internationally as a viable means of day atmosphere and is a critical compo- of waterflood resulted in oil production long-term CO2 storage. The International nent in models of global climate change peaking at 46,000 barrels/day in 1965. Energy Agency Weyburn CO2 Monitoring (IPCC, 2001). It is estimated that ~6 gi- Waterflood has continued since then, and Storage Project is designed to inves- gatons of carbon enters the environment with horizontal infill drilling commenc- tigate the technical and economic fea- annually as a result of global energy-re- ing in 1991. Approximately 24% of the sibility of CO2 storage in a partially de- lated CO2 emission, with North America original oil in place had been recovered pleted oil reservoir in conjunction with being responsible for ~20%–25% of this by 2000 when CO2 injection began. enhanced oil recovery operations. Two total (International Energy Agency [IEA], Weyburn oil reserves reside within a key elements of the project are (1) the 2000). Recognition of the importance of thin zone (maximum thickness of 30 m) establishment of a regional geoscience CO2 emissions has stimulated research of fractured carbonates in the Midale framework as a means for prediction toward mitigation of CO2 effects as man- beds (Fig. 2) of the Mississippian of the long-term fate of injected CO2, dated under the Kyoto Protocol of the Charles Formation, which were depos- and (2) development and application United Nations Framework Convention ited in a shallow carbonate shelf envi- of geophysical/geochemical monitoring on Climate Change. ronment. The reservoir comprises two and verification methods to track the CO2 sequestration in geological res- intervals, an upper Marly dolostone spread of CO2 within the reservoir. To ervoirs is being evaluated internation- (0–10 m thick) and lower Vuggy lime- 3 date, 1.90 billion m of CO2 have been ally as a viable means of long-term CO2 stone (0–20 m thick) that are sealed by injected into the reservoir, the effects of storage and climate change mitigation anhydritic dolostones and anhydrites of which are imaged by the various moni- (EAGE, 2000). In 2000, the IEA Weyburn the Midale Evaporite. The Midale Marly toring methods. CO2 Monitoring and Storage Project unit ranges from chalky dolomudstone GSA Today; v. 14; no. 7, doi: 10.1130/1052-5173(2004)014<4:GGSIAO>2.0.CO;2 4 JULY 2004, GSA TODAY Figure 1. Three-dimensional map showing the location of the Weyburn study area in southeastern Saskatchewan. The dashed cube represents the area where the geoscience framework is being constructed. Lower inset shows the location of the International Energy Agency geoscience framework study area within the Williston Basin. to calcitic biofragmental dolostone with (8%–20%) and higher permeability (10 1986) and is parallel to the horizontal intervening thin beds of biofragmental to >300 millidarcy). The higher permea- well direction. The vertical stress at the limestone. The fractured, Vuggy unit in- bility within the Vuggy unit resulted reservoir level due to the lithostatic load cludes a lower, peritidal “shoal” se- in preferential recovery of oil from this is ~34 MPa as estimated from a density quence with common secondary unit during the waterflood stage of log, and the minimum horizontal stress (vuggy) porosity, and an upper shallow production. is ~18–22 MPa in this region (McLellan marine “intershoal” sequence dominated The dominant fracture set within the et al., 1992). by fine-grained carbonate sands. The reservoir strikes NE-SW as determined Midale Marly has relatively high porosity from core and imaging logs (Bunge, THE CO2 FLOOD (16%–38%) and low permeability (1 to 2000). This orientation is sub-parallel to The CO2-based enhanced oil re- >50 millidarcy), whereas the Midale the regional trajectories of maximum covery (EOR) scheme was initiated in Vuggy has relatively lower porosity horizontal stress (Bell and Babcock, Figure 2. A: Simplified regional hydrostratigraphy above the Frobisher sequence. The CO2 is conceptually shown migrating up-dip within the Midale beds, which subcrop at the Mississippian unconformity and the overlying Watrous aquitard. B: Representative schematic distribution of major lithologies within the Midale reservoir with approximate location indicated by the red rectangle in A. The trajectory of a horizontal (Hz) injection well is shown within the Midale Marly unit. GSA TODAY, JULY 2004 5 September of 2000 in 19 patterns1 of the EnCana Weyburn unit at an initial injection rate of 2.69 million m3/day (or 5000 tonnes/day). The present rate of CO2 injection is 3.39 million 3 3 m /day of which 0.71 million m /day is CO2 recycled from oil production. The CO2 EOR is contributing over 5000 barrels/day to the total daily production of 20,560 barrels/day for the entire Weyburn unit. As of May 30, 2003, cumulative CO2 injected was 3 1.90 billion m . The CO2 flood will be expanded gradually over the next five years into a total of 75 patterns with ~10.8 billion 3 m (or ~20 million tonnes) of injected CO2 anticipated over the lifetime of the project. The source of CO is the Dakota 2 Figure 3. Subcrop and isopach of Gasification Company’s synthetic fuel plant located in Beulah, the reservoir and upper seal. North Dakota. The CO2 is transported 320 km via pipeline to the Weyburn field. The reservoir is at ~1450 m depth in the EOR area and has a mean temperature of 63 °C. Estimated pore pressures of ~14 MPa existed when the field was originally discovered, and during waterflood they ranged from 8 to 19 MPa. Recently mea- sured pore pressures range from 12.5 MPa to 18 MPa with an average of ~15 MPa. These conditions exceed the critical point pressure and temperature (7.4 MPa and 31 °C) of CO2, and thus the injected CO2 initially exists as a supercritical fluid in the reservoir. It is anticipated that the injected CO2 will cause minor dissolution of carbonate minerals, as CO2 will dissolve to some extent in water present in the reservoir, forming bicarbonate until equilibrium is reached. This is ionic trapping of CO2. The presence of minor amounts of silicate minerals within the reservoir may also enhance the capacity of the reservoir to sequester CO2 through mineral trapping as reactions between the silicates and CO2 may lead to the precipitation of carbonate minerals. THE REGIONAL FRAMEWORK The motivation for developing the regional geological framework is to establish a means for predicting whether injected CO2 may migrate beyond the immediate injection site over the long term. To address this requirement, key elements of the regional framework that must be identified include potential fluid pathways, including faults, fracture zones and controlling depo- sitional features, such as zones of salt dissolution. CO2 traps and trapping mechanisms (e.g., seals and aquitards) within the Figure 4. A representative two-dimensional seismic profile from the stratigraphic column must be identified as well, and the trans- 1600 km of profiles that are being used with geophysical and core port properties of deep aquifers (e.g., porosity, permeability, logs to construct the regional geological framework. Horizons that are fracture distribution, formation-water flow rates and directions) picked regionally include the Precambrian unconformity, Deadwood, Winnipeg, Winnipegosis, Prairie Evaporite, Upper Backen, Upper need to be established. Watrous, Manville, Lower Colorado, and 2nd White spec.

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